Ignitrons are high current switches that can open and close very quickly by using plasma (vaporized metal) arcs to complete the circuit. Typically the metal used is mercury because mercury does not readily plate on the inner surface of the ignitron. However, mercury is considered a hazardous and toxic substance, and attempts in the art to use non-hazardous metals, such as gallium, have not been successful because of the propensity for these metals to plate out and complete a circuit between an anode and a cathode.
Plating out is an undesirable occurrence which, once a continuous path of solidified conductive material, such as gallium, is formed along the inside surface of an ignitron's housing, shorts or permanently closes the switch, rendering the switch inoperable.
Ignitrons typically include a liquid metal cathode and an anode separated by a distance in a vacuum chamber. As current moves through an ignitor electrode, a small ignitor arc is formed between the ignitor electrode and the liquid metal surface forming the cathode, resulting in the vaporization of a quantity of the liquid metal. As the quantity of vaporized liquid metal increases, and if the potential difference between the cathode and anode is above a threshold level, the vaporized liquid metal completes the main circuit and a primary plasma arc is formed between the cathode and the anode, closing the ignitron switch and allowing current to flow.
Ignitrons are capable of conducting high currents, allowing for fast discharge of capacitors and providing high instantaneous power over a very short time. Ignitrons have been commonly used in pulsed lasers, pulsed fusion and power rectification. These and other pulsed power systems used by NASA require ignitrons because they are capable of conducting high currents and holding off high voltages. Typical switches are not practical for high current/high voltage pulsed systems because the switches cannot turn on or off quickly enough or conduct enough current.
One exemplary use for ignitrons is in present terrestrial power delivery systems. For example, solar storms may disrupt the Earth's magnetic fields, causing geomagnetically induced current to be produced in power lines. To prevent damage to electrical transmission equipment, capacitors may be used at the neutral-to-ground junctures. However, it is necessary to provide a way to quickly bypass the capacitor in the event of an actual fault to allow large current to flow from the neutral to the ground. Ignitrons may be useful in this application because they close more quickly than a mechanical switch and have the ability to handle high currents repetitively.
Ignitrons form a relatively short-lived electrical connection between two electrodes through a plasma arc composed of vaporized liquid mercury. Ignitions known in the art generally use mercury because it is a liquid at room temperature and perhaps more importantly, it does not rapidly plate out, or condense and form a solidified surface on the inside insulating surface of an ignitron's housing.
The main disadvantage with using mercury in ignitrons is its hazardous nature. The ignitron may not be cleaned (for example, when the mercury plates out) or otherwise serviced because mercury cannot be easily handled. Further, mercury-filled ignitrons must be specially disposed of, resulting in additional disposal costs.
There is an unmet need for ignitrons which are structurally adapted to use gallium, gallium alloys and other non-hazardous, non-toxic liquid metal in place of mercury, resulting in switches that are easier to handle, serviceable and reusable. Gallium and gallium alloy switches are also easier to dispose of, resulting in lower disposal costs.
There is also an unmet need for a gallium-based ignitron because gallium-based ignitrons hold off significantly higher voltages than mercury-based ignitrons due to the much lower vapor pressure of gallium. Gallium and gallium alloys are therefore ideal replacements for mercury in ignitrons.
U.S. Pat. Nos. 3,462,573, 5,478,978 and 5,792,236 teach switches where gallium was used in place of mercury. However, these switches use liquid gallium to complete or interrupt a circuit without vaporizing the gallium and forming a plasma. These switches, therefore, are not useful for high voltage and/or high current switching. The switches disclosed in these patents also do not consider vaporizing and ionizing the liquid gallium, with the current only traveling across a purely liquid conductive path.
Even if the gallium was vaporized in the switches described in U.S. Pat. Nos. 3,462,573, 5,478,978 and 5,792,236, the switches would be impractical because gallium and gallium alloys have a high disposition for plating out.
Gallium and gallium alloys plate out more readily than mercury, rendering switches that rely on vaporizing gallium or gallium alloys inoperable over a short period of time. Ignitrons using gallium and gallium alloys would therefore have a significantly shorter life span than the mercury-based ignitrons.